This invention relates to methods for reducing rotation components in table translation and, more particularly, to techniques for aiding in accurate image projection as required in scanning projection printing.
The copending application of M. Feldman and M. C. King, U.S. Ser. No. 277,275, filed Aug. 2, 1972, now U.S. Pat. No. 3,819,265, issued June 25, 1974, and assigned to Bell Telephone Laboratories, Incorporated, describes a photolithographic printing technique particularly useful in the fabrication of semiconductor devices. A semiconductor wafer covered with a photosensitive film and a mask of the circuit to be made are mounted on a common movable translation table. Only a small portion of the mask is imaged on the film by a high-resolution, small image field optical system, as is required for printing accurately the detailed mask pattern. The translation table is then reciprocated in an x direction and periodically stepped in a y direction to give raster scanning of the sensitized wafer by the projected mask image. This permits printing of an entire mask pattern through the use of a lens system having an image field area much smaller than the area of the pattern to be printed, thereby giving higher resolution than could ordinarily be obtained.
As the table reciprocates back and forth, it is important that the relative positions of the mask and wafer be maintained with an accuracy corresponding to the fine dimensions of the pattern being printed. Any spurious rotations of the table will result in a misalignment of the wafer with respect to the mask, with consequent blurring of the printed image. To take best advantage of the optical capabilites of the Feldman et al. technique for printing detailed integrated circuit patterns, we have determined that table rotation about a vertical or z axis should be maintained at less than .+-. 0.2 arc seconds.
The best mechanical apparatus presently available for insuring precise linear motion is an air bearing in which a movable element or sleeve surrounds an axial stationary element or stator and is separated from it and supported by a cushion of air. The sleeve and stator may be either cylindrical or rectangular in cross section, depending upon whether one wishes to permit angular movement of the sleeve about the central axis. In either case, a table mounted on the sleeve will move back and forth along the central axis with little friction and with more accuracy than that provided by any other known bearing. However, where the translation distance is on the order of inches, air bearings have been found to manifest a rotation component of about one arc second at best, which is insufficient for best operation of the scanning projection printing apparatus of the Feldman et al. application.